As used herein, the term "fine cutting" is intended to describe a high-precision stamping or punching of articles from sheet material, especially metal objects from metal strip, utilizing a punch having the profile of the article to be produced, and a cutting plate or die plate having an opening of complementary profile.
While generally punching and stamping are coarse processes capable only of producing blanks or articles which may be further shaped and which may deviate significantly in size or shape from the desired article, owing to deformation of the material to be punched, the inability to effectively control distortion during the punching or stamping operation, and other factors in the operation, a high quality punching and stamping process and tool has been developed for what can be described as fine stamping or fine punching.
In the latter system articles can be shaped to narrow tolerances because the workpiece is firmly held all around the hole and the punch during the step of driving the material into the hole which constitutes the punching operation.
In such a system a guide plate is juxtaposed with the die plate or cutting plate, the material, e.g. steel strip of a thickness of 0.3 millimeters to about 15 millimeters is fed between this guide plate and the die plate, the two parts clamp the material between them. A punch shiftable in the guide plate is displaced in registry with a hole of predetermined size and shape complementary to the size and shape of the punch, to drive the material through the hole and shear it from the strip. To support the material against distortion while it is being pressed through the hole, the article is supported during the stamping operation by an ejector which can have generally the same shape as the punch and between which the article is clamped during the displacement with the article from the strip. In other words around the perimeter of the hole, the material is supported by clamping it between the two plates while within the outline or perimeter of the hole, the material is supported by clamping it between the punch and the ejector.
The guide plate and/or the die cutting plate can be provided with an annular tooth or barb which can penetrate into the strip close to the periphery of the hole to prevent lateral displacement of the material during the stamping operation.
The resistance exerted by the ejector to the displacement of the stamping ram can amount to 10 to 20% of the cutting force which must be developed by the punch. This ensures that from the initial displacement of the material, the portion of the material which is adapted to form the article remains firmly clamped between the punch and the ejector.
The principles of fine cutting, the state of the art relevant thereto and the guide lines involved will be apparent from the publication VDI 3345 "FEINSCHNEIDEN" May 1980 (VDI Guidelines).
Because of the close tolerances required between the punch and hole in the die plate, generally the punch does not enter this hole and on the edges of the workpiece turned toward the punch thin burrs may remain. The bore in the guide plate in which the punch is displaced generally also has the same shape and size as the hole into which the article is forced in the punching operation.